Following completion of an experiment using neutron-rich RIBs in October,
the RIB injector platform was reconfigured for the installation and testing
of a multi-target batch-mode target/ion source for the production of
long-lived radionuclides. The target wheel was loaded with four calcium
fluoride targets for producing
18F
via the
19F(p,pn) reaction, and four nickel targets
for producing
56Ni
via the the
58Ni(p,p2n)
reaction. Testing of the
batch-mode target/ion source began in November 2000 and was completed in
January 2001. The results of the tests were very encouraging and measured
18F
beam intensities were within a factor of 4-5 of the predicted values.
The major cause of the lower intensity is believed to be due to the ORIC
beam spot being significantly larger than the sputtered area.
Experiments using this source have been postponed until the
18F
beam intensity can be increased and the users have developed
techniques to cope with the large
56Co
contaminant which will accompany the
56Ni
beam.

The batch-mode assembly was removed in early February 2001, and the RIB
injector is being readied for installation of a UC-target/EBP ion-source for
continuation of the n-rich RIB research program. The tandem electrostatic
accelerator was shut down for scheduled maintenance in mid-January and is
presently scheduled to resume operation the week of March 12.
Conditioning of the tandem and operation with stable-ion beams is
scheduled through mid-March to ensure that the tandem accelerator will
be ready to operate at up to 24 MV as required for the n-rich RIB
research program.

The n-rich RIB research program is presently scheduled to run from
mid-March, following the ISOL'01 meeting, until late April. During this time,
we expect to complete most of the approved n-rich RIB
experiments. Following the n-rich research period, a kinetic-ejection
negative-ion source, coupled to a hafnia target, will be installed on the
RIB injector platform. This target/ion source combination will be similar to
that used for the highly successful
17F/18F
research program completed last year. During installation of the RIB
target/ion source, the tandem accelerator will be available for
stable-ion beam experiments in April-May. Operation with
17F/18F
beams is presently planned to begin in mid-May and continue through June.
Following the
17F/18F
campaign, the accelerators will be shut down for scheduled maintenance.

The recent advances in experimental nuclear physics make it possible
to study nuclear systems far from the beta stability line. In these
nuclei the residual interaction between discrete and continuum levels
is expected to play a crucial role and cannot be neglected.
The Shell Model Embedded in the Continuum (SMEC) is a new formalism
that can be used to study both static properties (e.g. excitation
spectra, radii, quadrupole moments) of exotic nuclei as well as
simple reactions in which they are involved [1].
The SMEC is mainly based on the Continuum Shell Model [2], with
a more realistic description of the discrete levels, an essential
condition for the study of low-lying excitations.
In the SMEC formalism, the atomic nuclei is considered as an open
quantum system, i.e. the subspaces Q of (quasi-)bound
states and P of scattering states are not separated artificially.
In our case, the P subspace contains the states
with N-1 particles in bound orbits coupled to one particle that
can occupy
scattering states. We only consider states with asymptotically at most
one particle in the continuum, so the projectors
on the subspaces fulfill the condition: P+Q=1.
The key element of our treatment is the realistic description of the
discrete levels. We solve the standard Shell Model (SM) problem:

HQQ Pi =
EiPi

where QHQ=HQQ
is identified with a realistic SM Hamiltonian.
The eigenstates Pi
are the N-particle (quasi-)bound wave functions.
We believe that for a quantitative description of the low-lying
states in the exotic nuclei one has to use as a starting point the
accurate many-body wavefunctions provided by the SM with effective
interactions.
For the continuum states, we solve the coupled channel equations:

(E(+)-HPP)FE(+)=0

where c denotes the different channels and
HPP=PHP.
The superscript (+) means that boundary conditions for outgoing
scattering states are used. The solutions of these equations
are the nonresonant part of the scattering states. The couplings
between channels depend on the structure of the N and N-1
particle system and are determined microscopically.
The third system of equations to be solved consists of the
inhomogeneous coupled channel equations:

(E(+)-HPP)Wi(+)=HPQPi

in which the source term is primarily given by the SM structure
of the N-particle state
Pi.
The solution of this equation represents the
continuation of the discrete state
Pi
in the P subspace, and
describes the fact that a discrete level can decay by emitting a nucleon.

The three distinct functions are then used to express the many-body
scattering solutions of the problem and the discrete states
wave functions modified by the couplings to the continuum.
The latter are related with the effective
Hamiltonian

HQQeff=HQQ+HQPGP(+)HPQ,

where
GP(+)
is the Green function for the motion of single particles
in the P subspace. This effective Hamiltonian has complex
energy-dependent eigenvalues
Ei-iGammai/2.
These eigenvalues at energy
Ei(E)=E
determine the energies and widths of the nuclear states.

Fig. 2-1 - In the left part of the figure, the astrophysical S factor
for the reactions
16O(p,gamma)17F
[Jp=5/2+]
and
16O(p,gamma)17F
[Jp=1/2+]
are plotted as a function
of the center of mass energy. The parameters of the residual
interaction between P and Q and the effective interaction
in Q are discussed in [3]; the
experimental values are taken from [4]. The right part
represents the phase-shifts for the
p+16O
elastic scattering as a function of the proton energy for
different partial waves. The experimental points are from [5].

This formalism is fully symmetric and consistent in treating the
continuum and discrete part of the solutions,
the continuum states being modified by all the discrete states, and
the discrete levels being modified by the coupling to the continuum.

The SMEC has been applied for the study of the properties
of the mirror nuclei
8B
and
8Li,
and the radiative capture reaction
7Be(p,gamma)8B,
which is the key reaction for the understanding of the solar high
energy neutrino flux [1].

As an example, we report here some results concerning the study of
17F
and the reactions of elastic scattering
p+16O
and radiative capture
16O(p,gamma)17F.
The spectroscopic properties of
17F
reported in [3,6]
are well reproduced by our model. We have shown that a realistic
description of the structure of
16O
and
17F
(including the 2 particles - 2 holes correlations)
are required in order to reproduce in a consistent way both the
spectroscopy of
17F,
the
16O(p,gamma)17F
astrophysical S factor, and the phase shift of the elastic scattering
p+16O (see Fig. 2-1).

The SMEC give the possibility to investigate the static properties
of exotic nuclei and the reactions in which they are involved
within a single microscopic formalism. It can easily be used
for other reactions not discussed here, such as inelastic
scattering or proton emission. An extension for systems with
more than one particle in the continuum, although not straightforward,
is possible. We are currently working on the description of
18Ne
and reactions involving
17F.
We are also revisiting the radiative capture reaction
7Be(p,gamma)8B
by taking into account the influence of the target excitations.

References

*This work is being developed by a collaboration of researchers
from GANIL, the Institute of Nuclear Physics of Krakow, the
Laboratory of Theoretical Physics of the University of
Strasbourg, the University of Tennessee, and the Oak Ridge
National Laboratory.

We have started to investigate the feasibility of using the 25 MV Tandem
from the HRIBF for Accelerator Mass Spectrometry (AMS). AMS is one of
the analytical techniques with the highest sensitivity known in physics.
The technique of AMS is used to perform measurements of rare isotopes
in samples placed in the ion source of an accelerator system. AMS uses
a particle accelerator in conjunction with ion sources, magnets and
detectors to separate out interferences and count single atoms in the
presence of up to
1 x 1016
stable atoms. These radionuclides can be used for a wide variety
of dating and tracing applications in environmental and biological
monitoring, in the study of ocean circulation patterns, in radioactive
waste, nuclear safeguards and nuclear physics. One of the best known
applications is radiocarbon
14C
dating. Two features make the tandem accelerator the preferred
instrument for AMS. First, the requirement of negative ion beams for
injection eliminates the interference of some stable isobars
(14N,
36Ar,
129Xe)
which do not form negative ions that would impair the detection
of the radioisotopes of interest
(14C,
36Cl,
129I).
Second, the stripping of electrons at the high voltage terminal
eliminates molecular interference.

The most interesting radioactive nuclear beams (RIBs) produced
for astrophysics and nuclear physics involve generally rather
short-lived species such as
17F
(T1/2=64.5 s) and
18F
(T1/2=110 m).
By contrast, one of the primary objectives of AMS is the measurement
of long-lived radioisotopes produced in natural materials by the
interaction of cosmic rays (half-lives from a few years to millions of
years). At first sight it would seem that AMS and RIB production are very
different fields. However, there are a number of similarities between AMS
and RIBs. The removal of interfering isobars is one of several
common challenges of both AMS and RIB production. A facility such
as HRIBF has a variety of equipment choices for beam transport
and analysis and for rejection of unwanted species. Both AMS and
RIBs will benefit from the use of the most efficient techniques for
production, isobar separation, transport, and detection.

The HRIBF Tandem Accelerator is by far the highest operating voltage
electrostatic accelerator in the world. Tandem voltages up to 25.5 MV
have been used in experiments. The HRIBF 25 MV Tandem Accelerator
is capable of producing beams of 0.1-10 MeV per nucleon for light
nuclei and up to 5 MeV per nucleon for mass 80. There are a number
of isotopes of interest for AMS where the high energies achieved
could represent a very important advantage for their detection.
For example, the beam energies are sufficiently high to strip all
electrons from a good portion of the ions up to about mass 50.
Isobars are then produced in different charge states
that can be separated by beam line analyzers. This guarantees a strong
suppression
(107)
of background events originating from lighter isobars.
Furthermore, the HRIBF offers a variety of equipment such as high-resolution
momentum analyzers (including one internal to the machine), electrostatic
analyzers, plus a vast array of particle detection devices. We have started
to develop additional detection systems oriented to heavy isotopes. The
higher energies achieved will facilitate the isobar separation for the heavy
species. The various research tasks include the establishment of beam
diagnostic procedures for the low- and high-energy ends of the machine and
the study of the stability and reliability of operation of the tandem for
extremely low beam intensities.
In particular, the ion source, beam optics, terminal voltage stabilization,
and detection systems are extremely important in AMS.

Specifically we are interested in the potential use of the facility
for the detection of isotopes such as
36Cl,
44Ti,
90Sr,
99Tc,
129I, and
236U.
Very recently, an AMS experiment was performed at HRIBF to detect
36Cl,
for the first time in a sea water sample from the Scotia Shelf in
Nova Scotia, Canada. Also groundwater samples from the Great
Artesian Basin (the largest freshwater aquifer in Australia)
taken from a well known as "Oodnadatta" and a salina sample
from a drilling core from the "Salina Formation" in southwestern
Ontario, Canada. The samples in the form of AgCl were placed in
the ion source. The tandem was set at 22.5 MeV. The
Cl7+
were selected and postripped with a 50 ug/cm2 carbon foil to
Cl17+.
The ions were then transported to the Enge Spectrometer and
deflected to a silicon position sensitive detector in the focal plane.
These initial results proved that the HRIBF Tandem is
potentially the most powerful in the world for the measurement of
chlorine-36.

The 25 MV Tandem Accelerator of the HRIBF offers a powerful tool for AMS.
The necessary elements exist at ORNL to explore the establishment of an AMS
program: 25 MV Tandem, negative ion-source technology, accelerator
physics, mass spectrometry, detector development, and research interests.
We will focus on areas where the high voltage of the tandem plus
the specialized instrumentation we have (or will develop) can play a
unique role in AMS. Our proof-of-principle tests and our results will
indicate if it is possible to use HRIBF as a prototyping facility to
aid in the development of new AMS methods. This work is currently supported
by an ORNL SEED Money Award.

The ISOL'01 Conference will be held March 11-14 in Oak Ridge, TN.
Focusing on nuclear physics studies with radioactive ion
beams at ISOL facilities, the conference will have over 50 oral
presentations and one poster session. In addition, ISOL'01 is
dedicated to the celebration of twenty years of science at the
Holifield facility and its continuing leadership role
in nuclear physics research. More information including our
on-line registration form may be found on our web site at
http://www.phy.ornl.gov/isol01/.

The sixth Program Advisory Committee meeting has been scheduled
for June 14-15, 2001. Proposals for experiments will be due
on May 4 and must be submitted by email attachment to the address
liaison@mail.phy.ornl.gov.
Proposals should be targeted to the radioactive ion beam sources which
will be used in the last half of the year. At present, this would
involve the neutron-rich LaB6 source
targeted at isobaric contaminate-free beams of Br and I.
More information, including other ion sources to be scheduled
during this period, will be sent out in April. Approximately 100
shifts (8 hours) of RIBs and 100 shifts of SIBs will be allocated.

Initial tests with a uranium carbide target and a
LaB6
negative surface ionization source were reported in the last
newsletter. We have since done further tests to determine the
yields of radioactive isotopes of bromine and iodine after the
source was on for an extended period of time. The results
were favorable and showed that there were negligible changes in
the RIB intensities after a week of source operation. The
expected beam intensities into the Tandem for neutron-rich
isotopes of bromine and iodine are shown below. Remember, a
big advantage of this ion source over the EBP ion source is the
chemical selectivity. The beams shown below will be essentially
pure since the neighboring elements have little or no efficiency
for negative ion formation in this source. Beam on target is
typically 10% of what goes into the tandem for single stripping
and 1% for double stripping.

Isotope

Half-life

Expected beam into Tandem
(ions/second)
(with 10 uA of protons)

83Br

2.4 h

6x107

84Br

31.8 m

5x107

85Br

2.9 m

3x107

86Br

55.1 s

1x107

87Br

55.6 s

1x107

88Br

16.3 s

2x106

134I

52.5 m

2x107

136I

46.9 s

1x107

137I

24.5 s

2x106

138I

6.5 s

2x105

Aluminum RIB Development

Proton-rich radioactive isotopes of aluminum can be produced
in a silicon carbide target using the
28Si(p,alpha)25Al and
28Si(p,2pn)26Al
reactions. Using a production beam of 40 MeV protons, the
production rates in SiC are
2.6x10-425Al
atoms per incident proton and
5.6x10-426Al
atoms per incident proton. Silicon carbide has a low vapor
pressure at temperatures below the dissociation temperature
of 1670 C and works well in an Electron Beam Plasma (EBP)
ion source up to that temperature. This was demonstrated over
a period of several days using an EBP ion source coupled to a
thin-fiber (15 micron diameter) target of silicon carbide
held at temperatures up to 1650 C.

To measure the release of
25Al
(T1/2= 7.2 s)
from the SiC target, an on-line run was performed using a
low-intensity proton beam from the Tandem to bombard a target
at the UNISOR facility. Due to problems with the source, no
25Al
was observed. Since published diffusion rates of aluminum in
silicon carbide vary widely, we decided to use a SiC target in
powder form having an average particulate diameter of 1 micron
to provide short diffusion lengths. At some point, possibly
during the pump-down cycle, some of the SiC powder migrated
into the Ta transfer line between the target and the ion source.
This transfer line operates at 1800 to 2000 C so the SiC in
this region dissociated completely causing a large amount of
silicon to be released, destroying many of the Ta source parts
and 'killing' the ion source efficiency.

There were, however, some positive aspects to this experiment.
First, most (>90%) of the SiC powder was still in the target
holder and had not sintered. Second, while there was evidence
that the production beam did irradiate the target, no
26Al
(T1/2= 7.1x105 years)
was observed in the remaining SiC target material after the
irradiation. This is evidence that the diffusion rate of aluminum
in silicon carbide may be reasonably fast since the target
temperature was reduced to less than 1000 C within two minutes
after the end of the irradiation, effectively trapping any
26Al
that remained in the target at that time. Some
26Al
was found in the TIS enclosure deposited on cooler surfaces.

The next on-line test to look for the release of Al isotopes will
be done with a SiC fiber target in order to eliminate problems
inherent with a powder target. Even though the release efficiency
of short-lived isotopes from the fibers (15 micron diameter) is
likely to be lower than from the powder, we should be able to
extract enough beam to optimize the ion source parameters.
In parallel, a design for the target holder will be developed to
accommodate the fine powder targets, since powders (if they do not
sinter) have the largest surface to volume ratios.

ORIC was in scheduled shutdown mode for most of the reporting period. In
January, ORIC delivered up to 5 uA of 42 MeV protons for development of
beams with the multi-target batch-mode target/ion source. Calcium fluoride
and nickel targets were bombarded. Details of the RIB production are
presented in another section.

Tandem Operations and Development

The Tandem Accelerator has provided more than 657 hours of
beam on target since the last report. The machine ran at
terminal potentials of 4.92 to 22.46 MV and beams of
1H,
12C,
32S,
35Cl,
36Cl,
48Ti,
50Cr,
and 58Ni
were provided. The tank was opened two times during this period;
once to reestablish communication to the lower major dead section
and once due to a failed belt on the 25 kVA terminal Georator.
When the belt failed, it was decided to move up the scheduled maintenance
period since the RIB source needed to be changed. The tank was opened
on January 26, and we hope to close it by March 2. During this
maintenance the improved recirculation turbo pump for the gas stripper
will be installed.

In the last newsletter, we reported that a shorting strap
was removed from three tube sections which had been shorted for
more than a year, and magnets were placed to try to alleviate any
electron leakage into these tubes. The first conditioning
period showed that these tubes were now able to be conditioned
easily and to hold that conditioning for a long period of time.
It is not known whether the magnets cured the problem or the problem
went away with time. In any case, we now have the full column available
and could have operated at 24+ MV before the current tank opening,
but no experiment needed the corresponding energy. It required
132 hours of conditioning to reach this level, and we should be able
to return with a minimal amount of conditioning after the tank is
closed. The accelerator was conditioned to high voltage in anticipation
of higher energies needed for neutron-rich beams.

New Alpha Omega oxygen monitors have been installed in place
of the Beckman monitors, which had required a great deal of maintenance
throughout the years. The new monitors will require very little
maintenance and should not cause as many false alarms.

RIB Injector Operations and Development

The major effort during this reporting period has been the testing of the
batch-mode target ion source on the high voltage platform in C-111S.
The batch-mode target ion source consists of a remotely rotatable
eight-position water-cooled copper target wheel that can be held at -5 kV with
respect to the ionizer of a standard cesium sputter negative ion source.
One of the eight targets can be bombarded by ORIC beam to build up long-lived
radionuclides while, simultaneously, another target at 90 degrees can be
sputtered to deliver previously produced radionuclides to the tandem
accelerator. Four targets of
CaF2
(20% Cu) were interspersed with four targets of
natural Ni for
19F(p,pn)18F
and
58Ni(p,p2n)56Ni
production, respectively.
Our measurements with 5 uA of 42 MeV
1H
on a
CaF2/Cu
target for 1 hour
indicate that we would be able to deliver 2 million particles per second of
18F
to the tandem accelerator with a 2 hour bombardment and sputter cycle.
Since this beam intensity is not competitive with that which we were able to
obtain with the
HfO2
target and the kinetic ejection negative ion source, we are
investigating ways to increase the
19F output
(18F
output is proportional to
19F
output).
As of this writing, we have also started to examine the
58Ni
output of the batch mode target ion source (again,
56Ni
output is proportional to
58Ni
output).

A
microchannel plate detector
has been installed in the diagnostics box at the image of the
second-stage mass separator in beam line 12.
The focal area of this magnet is instrumented with a rotating wire beam
profile monitor, remotely operable horizontal and vertical slits, a Faraday
cup, a moving tape collection system, and now this new type of beam profile
monitor. This detector was developed as a transmission timing detector
for use at energies greater than 1 MeV-A.
However, we have found that it retains its excellent position sensitivity
when used as a stopped beam imaging device at 200 keV.
The device consists of a target foil that is inserted into the beam and a
stationary microchannel plate that detects the secondary electrons emitted
from the foil by the beam particles and is read out by a resistive layer
anode. Both the foil and the microchannel plate are backed by magnets
which guide the secondary electrons. The signals from the detector are
processed and displayed to yield a real time updating two dimensional
view of the beam. With a defocused beam incident on the object slits,
we saw spectacular images of both the object and image slits.
This new capability will help us understand the optics of the second-stage
mass separator and improve our ability to separate isobars injected into the
tandem accelerator.

The turbopump mounted on the beam line 9 diagnostics box providing the
upstream pumping for the target ion source failed.
Since it is necessary to pump on both sides of the target ion source (the
turbopump mounted on the quadrupole box immediately after the target ion
source provides the downstream pumping), the operating turbopump mounted on
the diagnostics box in front of the first-stage mass separator was moved to
the beam line 9 diagnostics box.
This arrangement provides adequate pumping capability pending repair of the
failed turbopump which was moved into the hood in RADLAB for bearing
replacement.
In addition to the turbopump work, the O-ring was replaced in the high vacuum
valve isolating beam line 9 from the target ion source and the actuator for
the viewer in the beam line 9 diagnostics box was removed.
All of this work proceeded smoothly despite having to be performed in full
anti-contamination clothing including respirators.
Even in the post uranium carbide regime (where contamination levels on the
extract electrode were 35 rad/hr (beta) a couple of weeks after
fission-product production), we are still able to effectively conduct
maintenance in C-111S.

The platform motor-generator shaft was overhauled because of excessive
vibration.
The platform and source motor-generators run continuously; the last shaft
overhaul for both was over two years ago.

During this reporting period we also replaced the air motor for the coupling
mechanism between the target ion source and beam line 9, moved the controls
for the roughing pumps in C111S to C111N, and installed a backing vacuum
accumulator for the turbopump in beam line 12 to minimize the running of its
associated roughing pump.
For the first time we remotely fixed a leak in the previously mentioned
coupling mechanism by adjusting (from C111N) the air pressure to its
pneumatic clutch.

A new vacuum chamber has been installed directly behind
the RMS target chamber and before the first quadrupole of
the RMS. The chamber is a 6-way, 8-inch inner-diameter
cross and mounts directly to the target chamber. This
arrangement will allow the placement of detectors and catcher foils
some 46 cm from the target. Such arrangements will be ideal in the study
of nanosecond isomers, prompt gating of reaction products with
a micro-channel plate plus thin foil detector, and possibly
time-of-flight measurements with other detectors. The present
micro-channel plate detector subtends approximately +/-2.5 degrees
with respect to the beam.

Fig. RA3-1 - The new 6-way cross located between the RMS target
chamber and the first quadrupole is shown. The distance from the
target to the center of the cross is approximately 46 cm.
At present, a micro-channel plate detector is inserted in the
upper section of the cross. The picture may be enlarged by "clicking"
on the image. Other images are also
available.

This new measuring position is available for user experiments. Space
constraints exist when used in combination with the CLARION array and
the cross is not compatible when the forward array (Si detector) part of
the HyBall is in place.

The new Users Executive Committee held their first telephone conference
call with the facility in mid-January. A brief status report of the facility
was presented and discussions ranged from the results of PAC-5, the
problems of isobaric beam contamination, and the location of the annual
HRIBF Users Meeting. A poll of the users has been conducted and it
has been decided to declare that the ISOL'01 conference, which includes a
tour of the HRBIF, will satisfy the annual meeting requirement mandated
in the Users Charter. We hope to return to our traditional annual
meetings at the DNP next year.

Other business conducted at the meeting was the election of the vice-chair
of the committee. We are happy to announce that Ani Aprahamian will
serve in this capacity for this year and will become chair of the
committee in 2002. Congratulations Ani!

HRIBF welcomes suggestions for future radioactive beam development. Such
suggestions may take the form of a Letter of Intent or an e-mail to the
Liaison Officer at
liaison@mail.phy.ornl.gov.
In any case, a brief description of the physics to be addressed with
the proposed beam should be included. Of course, any ideas on specific
target material, production rates, and/or the chemistry involved are also
welcome but not necessary. In many cases, we should have some idea of the
scope of the problems involved.

Beam suggestions should be within the relevant facility
parameters/capabilities listed below.

The tandem accelerates negative ions only.

Positive ions may be charge-exchanged or used directly off the
platform (E < 40 keV).

Typical reactions required to produce more than
106 ions per second are n, 2n, pn, and
alpha-n fusion-evaporation reaction channels and beam-induced fission
products. More exotic reactions are possible if extremely low beam
currents are all that is needed.

Species release is strongly related to the chemistry between the
target material and the beam species. It is best when the properties
are different and the target is refractory. Thin, robust targets
(fiberous, liquid metals, a few grams per square centimeter) must be
able to withstand 1500 degrees Celsius or more.

Minimum half-life is seconds unless chemistry is very favorable.

Very long-lived species (T1/2 > 1 h)
are probably best done in batchmode where sputter rates of the species
and target substrates are important.

Isobaric separation is possible for light beams (fully stripped ions),
while isobaric enhancement may be possible for heavy beams.